Ceramic powders are inorganic compositions that are naturally hydrophilic and require a coating to impart one or more of the following characteristics:
(1) A coating is required to enable the surface to be wetted by, or compatible with, organic materials such as solvents. Surface wetting is required to prepare an intimate ceramic powder/organic dispersion. PA1 (2) A coating is required to passivate the surface of the ceramic powder. PA1 (3) A coating is required to render anatase titania (TiO.sub.2) non-photoactive. PA1 (4) A coating is required where specific functionalities are required on the ceramic powder surface, such as epoxy groups, carboxyl groups, and the like. PA1 (5) A coating is required to form dispersed-phase, aqueous ionic-gels that do not phase separate. PA1 w is tetraethylorthosilicate; PA1 x is selected from the group consisting of .gamma.-glycidoxypropyltrimethoxysilane, n-hexyltrimethoxysilane, isobutyltrimethoxysilane, .gamma.-methacryloxypropyltrimethoxysilane, n-octadecyltrimethoxysilane, and n-propyltrimethoxysilane; PA1 y is selected from the group consisting of dicyclohexyldimethoxysilane, diethyldiethoxysilane, dimethyldichlorosilane, dimethyldiethoxysilane, dimethyldimethoxysilane, diphenyldiethoxysilane, diphenyldimethoxysilane, di-n-hexyldichlorosilane, n-hexylmethyldichlorosilane, methyldodecyldiethoxysilane, neophylmethyldimethoxysilane, and n-octylmethyldimethoxysilane; and PA1 z is selected from the group consisting of n-octadecyldimethylmethoxysilane, triethylsilanol, trimethylethoxysilane, and trimethylmethoxysilane. PA1 (a) polymerizing a tetrafunctional siloxane monomer and at least one of a trifunctional siloxane monomer, a difunctional siloxane monomer and a monofunctional siloxane monomer; PA1 (b) adding a quantity of ceramic powder to a purged reaction vessel; PA1 (c) shear mixing the ceramic powder for a time sufficient to wet substantially all of the powder surface; PA1 (d) adding the siloxane polymer prepared in step (a) to the reaction vessel containing the shear mixed ceramic powder; PA1 (e) shear mixing the shear mixed ceramic powder and the siloxane polymer for a time sufficient to form a siloxane polymer coated ceramic powder; PA1 (f) separating the coated ceramic powder from the components remaining in the reaction vessel. PA1 the tetrafunctional siloxane monomer is tetraethylorthosilicate; PA1 the trifunctional siloxane monomer is selected from the group consisting of .gamma.-glycidoxypropyltrimethoxysilane, n-hexyltrimethoxysilane, isobutyltrimethoxy silane, .gamma.-methacryloxypropyltrimethoxysilane, n-octadecyltrimethoxysilane, and n-propyltrimethoxysilane; PA1 the difunctional siloxane monomer is selected from the group consisting of dicyclohexyldimethoxysilane, diethyldiethoxysilane, dimethyldichlorosilane, dimethyldiethoxysilane, dimethyldimethoxysilane, diphenyldiethoxysilane, diphenyldimethoxysilane, di-n-hexyldichlorosilane, n-hexylmethyldichlorosilane, methyldodecyldiethoxysilane, neophylmethyldimethoxysilane, and n-octylmethyldimethoxysilane; and PA1 the monofunctional siloxane monomer is selected from the group consisting of n-octadecyldimethylmethoxysilane, triethylsilanol, trimethylethoxysilane, and trimethyl-methoxysilane. PA1 w is tetraethylorthosilicate; PA1 x is selected from the group consisting of .gamma.-glycidoxypropyltrimethoxysilane, n-hexyltrimethoxysilane, isobutyltrimethoxysilane, .gamma.-methacryloxypropyltrimethoxysilane, n-octadecyltrimethoxysilane, and n-propyltrimethoxysilane; PA1 y is selected from the group consisting of dicyclohexyldimethoxysilane, diethyldiethoxysilane, dimethyldichlorosilane, dimethyldiethoxysilane, dimethyldimethoxysilane, diphenyldiethoxysilane, diphenyldimethoxysilane, di-n-hexyldichlorosilane, n-hexylmethyldichlorosilane, methyldodecyldiethoxysilane, neophylmethyldimethoxysilane, and n-octylmethyldimethoxysilane; and PA1 z is selected from the group consisting of n-octadecyldimethylmethoxysilane, triethylsilanol, trimethylethoxysilane, and trimethylmethoxysilane.
As used herein, the term "ceramic" refers to metal oxides, including but not limited to titanium dioxide (TiO.sub.2 ; sometimes referred to as "titania"), alumina (Al.sub.2 O.sub.3), zinc oxide (ZnO), and iron oxides including .gamma.-Fe.sub.2 O.sub.3 (brown in color), .alpha.-Fe.sub.2 O.sub.3 (red in color) and ferrous oxide (Fe.sub.3 O.sub.4 ; black in color; sometimes referred to as "magnetite"), and nonmetal oxides, including but not limited to silicon dioxide (SiO.sub.2 ; sometimes referred to as "silica").
Inorganic surfaces have been conventionally modified by absorption, ion exchange, and covalent bonding. Surface modification by absorption and ion exchange require the surface to have appropriate chemical characteristics. Reactions that enable covalent bonding to inorganic surfaces generally involve reactions on hydroxylic surfaces.
Inorganic surfaces may also be coated by graft polymerization and encapsulation. Inorganic powders may be coated by the precipitation of powders in the presence of suspended powders or by spray drying of polymer solutions containing the powder. However, these conventional methods yield uneven coatings and the formation of coated agglomerates. Graft polymerization initiated by adsorbed species, or involving their copolymerization, favors uniform polymeric coatings.
The present siloxane star-graft polymer coatings are derived from the acid-catalyzed silicate sols discussed in Sol-Gel Science, C. J. Brinker and G. W. Scherer, Academic Press, 1990, Chapters 3 and 4. Such acid-catalyzed silicate sols are fractal, silicon-based polymers, the structure of which is shown, in two-dimensions, in FIG. 1. The present siloxane star-graft polymers employ this polymer morphology, in three dimensions, as a starting point, bonding to the fractal backbone specific moieties, thereby forming a fractal, star-graft copolymer using molecularly engineered inorganic surface/diluent interactions. Inherent in the present method of preparing such siloxane star-graft polymers is the control of the fractal nature of the backbone by allowing only desired branching probabilities to occur. Such control is realized by selection of monomers with the desired functionality and reactivity.
Silicon-based polymers will be referred to herein using the following nomenclature: EQU Si(w,x,y,z),
where w, x, y and z refer to the mole percent tetrafunctional, trifunctional, difunctional and monofunctional monomers, respectively, that are employed in synthesizing the sol. The ratio of total moles water to total moles silicon (H.sub.2 O/Si) is termed R, where R is a measure of the degree of polymer branching.